The massive multiple-input multiple-output (MIMO) technology has
inspired wide research-attention, since it is capable of dramatically
increasing the system capacity, hence mitigating the spectrum shortage
[1]}, [2]}, [3]}. This is achieved by forming
directional beams. However, these large-scale antenna arrays employed
both at the transmitter and receiver have a prohibitive hardware cost
and power consumption in fully-digital (FD) beamforming, where each
antenna element requires a dedicated radio frequency (RF) chain. Thus,
hybrid analog-digital (AD) beamforming has become an important
technique of reducing the number of RF chains, while still approaching the
performance of fully-digital beamforming [3]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}.
